In conventional reading systems in which the width or other dimension of a coded mark is important the depth of field problem has been met in a number of ways. In one approach a specific mark is used as a reference against which the sizes of the other marks are compared. However, this requires relatively complex equipment to identify the reference mark and to carry out the comparison. Further, if the marks and reader are moved toward or away from each other after the reference mark is sensed error may result. In another approach, in which pairs of contrasting coded marks are viewed as a set, clock pulses increment a counter during the interval of one mark and then the counter is decremented by clock pulses during the interval of the contrasting mark. If the remainder in the counter is positive the first mark is bigger, if negative the second mark is bigger. With this technique when the decision is made that one of the marks is the larger and the other the smaller it is assumed that the two marks differ in size by the proper ratio but in fact the ratio may be any that satisfies the condition that the one is larger and the other smaller.